detail of chlorine alkali based industry

8/12/2019 Detail of Chlorine Alkali Based Industry

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Detail of Chlorine-Alkali based industry

Introduction

Chlorine Alkali Industry is the industry that produce chlorine and

caustic(alkali). Chlorine can't be produced without having caustic as

byproduct and Vica Versa. For that reason, chloralkali industry

always have to battle to balance production to the !luctuating

re"uire#ent. chlorine and caustic de#ands never get along.Also the

ti#e it takes !or chlorine to being #anu!actured to being

used$consu#ed is very less co#pared to caustic which #akes #orei#balance.

%ne o! &eading co#pany !or ChlorAlkali industry is

lectrolysisis one o! the acknowledged #eans o! generating che#ical

products !ro# their native state. For e*a#ple, #etallic copper is produced by

electroly+ing an a"ueous solutiono! copper sul!ate, prepared by leaching the

copper bearing ores with sul!uric acid. %r, one can prepare chlorine gas andsodiu# hydro*ide solution by electroly+ing an a"ueous solution o! sodiu#

chloride, which e*ists in nature in a solid !or# as rock salt and also available

as solar or vacuu# evaporated salt. he solution o! sodiu# chloride (co##on

table salt) is o!ten called -brine.-

he pri#ary products o! electrolysis are chlorine gas, hydrogen gas, and

sodiu# hydro*ide solution (co##only called -caustic soda- or si#ply

-caustic-). owever, i! the electrolyteis #aintained at apo! /.0 or 12, one

can !or# chlorate or hypochlorite !ro# the electrogenerated chlorine and

caustic. his is the basis !or the electrolytic production o! sodiu# chlorate orsodiu# hypochlorite (co##only known as -bleach-).

Chlorine and sodium hydroxide end uses
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Chlorine and sodiu# hydro*ide are a#ong the top ten che#icals produced in

the world and are involved in the #anu!acturing o! a wide variety o! products

used in daytoday li!e. hese include3 phar#aceuticals, detergents,

deodorants, disin!ectants, herbicides, pesticides, and plastics.

he !irst observation o! a possible application !or chlorine was its bleaching

e!!ect on vegetable #atter. In 1445, Carl 6ilhel# investigated the reactivity o!

the greenishyellow gas generated during the reaction involving the o*idation

o! hydrochloric acid by a #anganese dio*ide ore (pyrolusite). In 1470,

8erthollet tried unsuccess!ully to use ele#ental chlorine !or te*tile bleaching

to replace solar bleaching. le#ental chlorine caused disco#!ort to the

workers, corroded#etal parts, and so!tened the !abrics. he !irst use o!

chlorine in the !or# o! potassiu# hypochlorite was !or bleaching, and dates

back to 1479. It was in 1727 that :avy characteri+ed this greenishyellow gas

as an ele#ent and na#ed it -chlorine.-

he develop#ent o! che#ical bleaching with chlorine and the discovery o!

calciu# hypochlorite bleaching powder as a practical #ode o! transporting

chlorine was o! great signi!icance. hese technologies #ade a #arked i#pact

on the te*tile bleaching operations in reat 8ritain and urope, who were in

the #iddle o! the industrial revolution with e*panding production, and hence,

the de#and !or te*tiles. he invention o! the power loo# provided the

capability to produce te*tiles on a large scale. owever, solar bleaching, by

spreading the cloth in open !ields !or #onths, beca#e increasingly e*pensive

in view o! the soaring land values. he chlorine bleaching process not only

shortened the operations !ro# #onths to !ew days, but also !reed vast areas o!

land !or #ore productive use. 8ased on the greatly i#proved e!!iciency o!

te*tile bleaching, the pulp and paper industry also began using bleaching

powder. 8etween 140/ and 19;


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beco#e a co##on practice. here was signi!icant reduction in the incidence

o! water borne diseases, such as typhoid. For e*a#ple, !ro# %ctober to

:ece#ber 1929, 059 cases o! -winter typhoid- were reported in =ontreal,

Canada. A!ter chlorination o! drinking water was begun in 1912, only 142

cases were reported !or the sa#e 5#onth period. hus, virtually all thechlorine #anu!actured during the 19th century was consu#ed by these two

industries. he #a>or turning event !or the growth o! the chlorine industry was

its use in 191< !or water puri!ication during the ?iagara Falls typhoid

epide#ic. It should be noted that bleaching powder was used in 1794 to clean

the polluted #ains during a typhoid break in ngland.

8etween 19


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water treat#ent operations as a disin!ectant (Figure 1). he other uses o!

chlorine include the production o! organic and inorganic che#icals. he

largest volu#e organic che#ical #anu!actured that involves chlorine is

polyvinyl chloride (VC). VC is a very versatile ther#oplastic, used in a

wide variety o! daily products. he #a>or use o! chlorine in the production o!inorganic che#icals is !or titaniu# dio*ide (a widely used pig#ent),

#anu!actured !ro# naturally occurring ores (il#enite or rutile).

he end uses o!

caustic (sodiu#

hydro*ide) are

diverse

co#pared to the

uses o! chlorine

(Figure or use o! caustic !or #aking inorganic che#icals is in the production

o! hypochlorite !or household and industrial bleaching purposes. Also, its use

in the pulp and paper industry is in the production o! sodiu# sul!ide and

sodiu# hydrosul!ide !or #echanical pulping. It is also used in the !ood

processing applications, which include skin re#oval o! potatoes, to#atoes etc,

!or !urther processing.

Fig. 2. Caustic soda end uses.

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Sodium chlorate end uses

he

electrosynthesis

o! sodiu#chlorate dates

back to 172


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he chloralkali (also

called -chlorine

caustic-) industry is

one o! the largest

electroche#icaltechnologies in the

world. It is an energy

intensive process and is

the second largest

consu#er o! electricity

(


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co#posite deposited on a !ora#inous cathode. In #e#brane cells, on the other

hand, an ione*change #e#braneis used as a separator. Anolytecatholyte

separation is achieved in the diaphrag# and #e#brane cells using separators

and ione*change #e#branes, respectively, whereas #ercury cells contain no

diaphrag# or #e#brane and the #ercury itsel! acts as a separator. he anodein all technologies is titaniu# #etal coated with an electrocatalyticlayer o!

#i*ed o*ides. All #odern cells (since the 1942's) use these socalled

di#ensionally stable anodes- (:@A). arlier cells used carbonbased anodes.

he cathodeis typically steel in diaphrag# cells, nickel in #e#brane cells,

and #ercury in #ercury cells. hese cell technologies are sche#atically

depicted in Figures 04 and are described below.

Mercury cells

he #ercury cell hassteel botto#s with

rubbercoated steel

sides, as well as end

bo*es !or brine and

#ercury !eed and e*it

strea#s with a !le*ible

rubber or rubber

coated steel cover.

Ad>ustable #etal

anodeshang !ro# the

top, and #ercury

(which !or#s the

cathodeo! the cell)

!lows on the inclined botto#. he current!lows !ro# the steel botto# to the

!lowing #ercury.

@aturatedbrine !ed !ro# the end bo* is electroly+ed at the anode to produce

the chlorine gas, which !lows !ro# the top portion o! the trough and then e*its.

he sodiu# ion generated reacts with the #ercury to !or# sodiu# a#alga#(an alloy o! #ercury and sodiu#), which !lows out o! the end bo* to a vertical

cylindrical tank. About 2.


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unreacted brine !lows out o! the e*it end bo*. @o#e cells are designed with

chlorine and anolyteoutlets !ro# the end bo*, which are separated in the

depleted brine tank. he #ercury !ro# the deco#poser is pu#ped back to the

cell.

Diaphragm cells

he diaphrag# cell is a

rectangular bo* with

#etal anodessupported

!ro# the botto# with

copperbase plates,

which carries a

positive current. he

cathodesare #etalscreens or punch plates

connected !ro# one

end to the other end o!

the rectangular tank.

Asbestos, dispersed as

a slurry in a bath, is

vacuu# deposited onto the cathodes, !or#ing a diaphrag#. @aturatedbrine

enters the anode co#part#ent and the chlorine gas liberated at the anode

during electrolysis, e*its !ro# the anode co#part#ent. It is saturated with

water vapor at a partial pressure o! water over the anolyte. he sodiu# ionsare

transported !ro# the anode co#part#ent to the cathode co#part#ent, by the

!low o! the solution and by electro#igration, where they co#bine with the

hydro*yl ions generated at the cathode during the !or#ation o! the hydrogen

!ro# the water #olecules. he diaphrag#resists the back #igrationo! the

hydro*yl ions, which would otherwise react with the chlorine in the anode

co#part#ent. In the cathode co#part#ent, the concentration o! the sodiu#

hydro*ide is D1


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In a #e#brane cell, an

ione*change

#e#braneseparates

the anodeand cathode

co#part#ents. heseparator is generally a

bilayer #e#brane

#ade o!

per!luorocarbo*ylic

and per!luorosul!onic

acidbased !il#s,

sandwiched between

the anode and the

cathode. he saturated

brine is !ed to the anode co#part#ent where chlorine is liberated at the anode,and the sodiu# ion #igratesto the cathode co#part#ent. Enlike in the

diaphrag# cells, only the sodiu# ions and so#e water #igrate through the

#e#brane. he unreacted sodiu# chloride and other inert ions re#ain in the

anolyte. About ;2;


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!uture e*pansions or replace#ents o! e*isting circuits is the #e#brane cell

technology. he #a>or #e#brane cell technology suppliers, include3 Ehde

#b, Asahi Che#icals, and Chlorine ngineers. :e?ora ech is the sole

supplier o! diaphrag# cell technology. It is highly unlikely that anyone will

build a new #ercury or diaphrag#cell plant in the !uture. Figures 7 and 9illustrate cell roo#s with diaphrag# and #e#brane chloralkali cells.

Fig. !. Chlor"al#ali cell room $ith %&"2. membrane

cells 'Courtesy of Uhde (mb)*.

Chlorine processing

he chlorine gas !ro# the anode co#part#ent contains #oisture, byproduct

o*ygen, and so#e back#igrated hydrogen. In addition, i! the brine is alkaline,

it will contain carbon dio*ide and so#e o*ygen and nitrogen !ro# the air

leakage via the process or pipelines.

Chlorine is !irst cooled to /2oF (1/oC) and passed through de#isters to re#ove

the water droplets and the particulates o! salt and sodiu# sul!ate. he cooled

gas goes to sul!uric acidcirculating towers, which are operated in series.Co##only, three towers are used !or the re#oval o! #oisture. he dried

chlorine then goes through de#isters be!ore it is co#pressed and li"ue!ied at

low te#peratures. he noncondensed gas, called sni!t gas, is used !or

producing hypochlorite or hydrochloric acid. I! there is no #arket !or

hydrochloric acid, the sni!t gas is neutrali+edwith caustic soda or li#e

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(calciu# hydro*ide) to !or# hypochlorite. he hypochlorite is either sold as

bleach or deco#posed to !or# salt and o*ygen.

Hydrogen processing

he hydrogen gas !ro# the chloralkali cells is nor#ally used !or the

production o! hydrochloric acid or used as a !uel to produce stea#. ydrogen

!ro# #ercury cells is !irst cooled to re#ove the #ercury, which is then

returned to the cells. %ccasionally, a secondary treat#ent is used to re#ove the

trace levels o! #ercury in the hydrogen via #olecular sieve colu#ns. he

hydrogen gas is then nor#ally co#pressed. I! a custo#er needs nearly pure

hydrogen containing low a#ounts o! o*ygen, so#e plants will heat the

hydrogen over a platinu# catalyst(to re#ove the o*ygen by reacting it with

the hydrogen to !or# water), cool, and co#press the diaphrag# or #e#brane

cell hydrogen, be!ore supplying it to the custo#er. he heat value in thehydrogen cell gas can be recovered in a heat e*changer via heating the brine

!eed to the cells. Although only shown in Figure 1; in the Appendi*!or

diaphrag# cells, the heat recovery !ro# hydrogen is also possible with

#ercury and #e#brane cell processes.

Caustic soda processing

Caustic soda is #arketed as 02, 4;, or anhydrous(dry) beads or !lakes.

he #ercury cell can produce 02 and 4; caustic directly. he caustic !ro#

the deco#poser is cooled and passed once or twice through an activatedcarbon !ilter to reduce the #ercury levels in the caustic. A!ter !iltration, the

#ercury concentration is lowered to the partsper#illion (pp#) levels. ven

these low levels o! #ercury #ay be unacceptable to so#e custo#ers, who then

have to switch to using #e#brane grade caustic soda. he #ercury cell caustic

soda has a !ew pp# salt and 0pp# sodiu# chlorate. he #ercury cell caustic

is the highest purity caustic that can be #ade electrolytically i! trace

concentrations o! #ercury are tolerable in the end use o! caustic.

he #e#brane cell caustic is concentrated in a #ultiple e!!ect !alling !il#evaporator, which increases the caustic soda concentration to 02 with a high

stea# econo#y. Caustic soda !ro# #e#brane cells generally has ;2pp#

sodiu# chloride and 012 pp# sodiu# chlorate.

he catholyte!ro# the diaphrag# cells contains D1


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chlorate. he catholyte is evaporated in a #ultie!!ect evaporator. =ost o! the

salt !ro# the catholyte will precipitate during the concentration o! the caustic

soda to 02 sodiu# hydro*ide. he 02 caustic soda product will contain

about 1 sodiu# chloride. he 02 caustic also has a high chlorate

concentration (D2.1) co#pared to the caustic !ro# #e#brane or #ercurycells (D12 pp#). he salt, separated !ro# the caustic during evaporation, is

used to resaturate the brine !ed to the cell.

An additional singlee!!ect evaporator is needed to produce 4; caustic soda.

Anhydrous (dry) caustic soda is produced in a rising !il# evaporator, operating

at 4


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he #e#brane cell process brine speci!ications are #ore stringent than that o!

the #ercury and diaphrag# processes, and calls !or i#purities to be at the

partsperbillion (ppb) level. his is acco#plished by !iltering the brine in a

precoat type secondary !ilter. An ione*change resinis used to re#ove the

calciu#, #agnesiu#, bariu#, and iron i#purities. It is also possible to re#ovealu#inu# by ion e*change. %!ten, alu#inu# and silica are re#oved by adding

#agnesiu# chloride in the brine e*iting !ro# the salt dissolver.

he depleted brine !ro# the #e#brane and #ercury cell processes carries

dissolved chlorine. his brine is acidi!iedto reduce the chlorine solubility, and

then dechlorinated in a vacuu# brine dechlorinator. he dechlorinated brine is

returned to the brine wells !or solution #ining or to the salt dissolver. I! the

#e#brane and diaphrag# processes coe*ist at a given location, the

dechlorinated brine can be sent to a saturator !or resaturation be!ore being sent

to the diaphrag# cells.

Sodium chlorate manufacturing process

%ne o! the energy intensive electrolytic industries is the production o! sodiu#

chlorate by the electrolysis o! sodiu# chloride solutions in an electrolytic cell

without a separator. he products o! the electrode reactions, the chlorine and

the caustic, are allowed to inter#i* and react, producing sodiu# chlorate as

the !inal product (see the Appendi*!or details).

he #a>or raw #aterial is sodiu# chloride, either very pure, such as solar rocksalt, or partially puri!ied evaporated salt. he salt is stored and dissolved in

li*iviators to produce a saturatedsodiu# chloride solution. his solution is

puri!ied by re#oving calciu#, #agnesiu#, !luoride, sul!ate, and iron as

insoluble co#pounds, through the addition o! sodiu# carbonate, sodiu#

hydro*ide, sodiu# phosphate, and bariu# chloride.

he i#purities or precipitates are re#oved in a pressure lea! !ilter with

diato#aceous earth as a !ilter precoat and !ilter aid. his !ilter cake, containing

appro*i#ately ;0 water, is the only solid waste strea# !ro# the process. Apolishing !iltration stage and an ione*change syste#!ollow pressure lea!

!iltration.

he che#istry and electroche#istry o! chlorate !or#ation dictates that an

e!!icient and econo#ical cell should e#body several distinct +ones. In the

electrolysis+one, the electrolytic reactions take place along with the hydrolysis

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o! chlorine. As the che#ical chlorate !or#ation proceeds very slowly, a

relatively large volu#e o! che#ical reaction +one is needed. A cooling +one is

also re"uired to re#ove the e*cess heat generated !ro# the reaction and

control the operating te#perature. he cooling +one #ay be located within the

che#ical reactor or in an e*ternal heat e*changer. ydrogen gas generated atthe cathode#ust be released !ro# the cell li"uor. his hydrogen release takes

place in the electrolysis cell, a separate vessel, or the che#ical reactor.

A continuous strea# o! cell li"uor !lows !ro# the electrolysis syste# to the

-hypo re#oval- syste#, where the sodiu# hypochlorite concentration is

reduced to low levels si#ply by heating the cell li"uor to about 170


15/28

Appro*i#ately 97 o! the sodiu# chlorate capacity in ?orth A#erica is

produced directly in sodiu# chlorate cells. he re#aining


16/28

he #arket de#and,

environ#ental

constraints, and energy

prices have pri#arily

dictated the growth o!the chloralkali

industry in the E.@.,

with a #a>or ;1

share o! the world

capacity. he industry

en>oyed a strong

growth until about

1942. It !altered in

1942's and peaked

brie!ly in 1949 be!ore!alling to the lowest

level during 197


17/28

bleaching. :uring this period, #any che#ical processes that used chlorine,

particularly ethylene o*ide and propylene o*ide, were also converted to non

chlorine consu#ing processes.

Chloralkali producers, ignoring the potential i#pact o! new nonchlorinebased technologies and the various environ#ental concerns, continuing to

pro>ect growth rates o! 5/$yr. hese pro>ections are based on chlorine

de#and !ro# e*ports, particularly to the Far ast. Anticipating signi!icant

growth in e*ports, 10,222 tons$day o! new capacity was added through the

early 1972's.

In the 1972's, the environ#ental constraints i#pacted the downstrea# use o!

chlorine and operating costs increased because o! the energy crisis or the cost

o! electricity. In addition, the e*ports declined because o! the new ethylene

dichloride (:C) plants co#ing on strea# overseas. As a result, the de#anddeclined and the industry operated at only a /5 capacity. %vercapacity, slow

growth, and high energy costs !orced chloralkali producers to #othball or put

on a standby #ode a large nu#ber o! production !acilities, accounting !or

about 1.< #illion #etric tons(=). 8y the end o! or !actors that in!luenced the chloralkali industry are the

environ#ental issues related to the use o! products such as :: or aerosols

and the develop#ent o! nonchlorine based technologies. here is yet another

proble# that con!ronts chloralkali producers, that is, the outo!phase de#and

!or chlorine and caustic soda. Chlorine #arkets !ollow the econo#y closely,

since a large portion o! the VC #arket (its largest application) is in the

housing and auto#otive industries.

Caustic soda, on the other hand, does not respond as readily to econo#ic

changes because o! the diverse nature o! its #arkets, such as pulp and paper

and che#ical processing. Another advantage !or caustic soda is that it can

#ore easily be stored which helps !latten out variable de#and. hese!luctuating de#ands !or chlorine and caustic soda, resulting !ro# the overall

changes in the econo#y, generally lead to production cut backs and increased

prices !or either chlorine or caustic soda.

In the late 1972's, the chlorine industry recovered !ro# earlier declines in

consu#ption and en>oyed banner production years. In 1974 and 1977, annual

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increases reached 5 to 0 due to the strong econo#y. his was characteri+ed

by the increased de#and !or VC and pulp and paper products and by

increased e*ports o! chlorine derivatives.

It should be noted here that #ost o! the chlorine is traded globally as :C,vinyl chloride #ono#er (VC=) and pri#ary !or#s o! VC and that very little

in its ele#ental !or#. he E.@. alone accounted !or al#ost 02 o! this trade in

199


19/28

chlorine dio*ide !or chlorine over total chlorine!ree bleaching, has spurred the

de#and !or sodiu# chlorate which is pro>ected to grow at a rate o! 0 per

year through the ne*t ten years.

Environmental issues within the chlor/alkali industry

here are several environ#ental concerns that have #ade a signi!icant i#pact

on the growth o! the chloralkali industry over the past twenty years and will

dictate the !uture growth as well. hese issues are highly debated, and the

associated -che#ophobia- is likely to adversely a!!ect the chlorine

consu#ption pro!ile in the !uture.

Chlorine bleaching of wood pulp and dioxin emissions to the

environment

resence o! dio*in, at parts per trillion (ppt) levels, in paper and paper based

products and chlorinated organics in pulp #ill e!!luents led to decreased

chlorine de#and. In the E.@., chlorine consu#ption in the pulp and paper

industry, decreased !ro# 10 in 1974 to 4 in 1997. he E.@. nviron#ental

protection agency pro#ulgated -Cluster Hules- in late 1997, #andating the use

o! ele#ental chlorine!ree bleaching. hese rules, which went into e!!ect in

April


20/28

In 1974, appro*i#ately ;7 o! all E.@. chlorine production was consu#ed in

vinyl chloride #ono#er (VC=) production to satis!y the growing polyvinyl

chloride (VC) de#and. hrough


21/28

Asbestos is used as a separator#aterial in diaphrag# cells. owever, asbestos

is a to*ic #aterial, causing lung cancer, asbestosis, and #esothelio#a. As a

result, in


22/28

Heaction J9K will conta#inate the caustic product with chlorate.

Chemical reactions occurring in brine processing

Comparison of cell technologies

ercury !iaphragm embrane

%perating current density(kA$#


23/28

Fig. 12. ,ercury cell process flo$ sheet.


24/28


25/28

Fig. 13. iaphragm cell process flo$ sheet.


26/28


27/28

Fig. 14. ,embrane cell process flo$ sheet.

$odium hypochlorite"chlorate manufacturing process

Electrochemical and chemical reactions occurring in cells

ypochlorite !or#ation is pro#oted by the use o! weak brine,basicsolution,

and low cell te#peratures.

Chlorate !or#ation is pro#oted by the use o! saturatedbrine, acidicsolution,

and te#peratures close to the boiling point o! the solution.

Chlorate cell process flow sheet

J1K


28/28

Fig. 15. Chlorate cell process flo$ sheet.
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detail of chlorine alkali based industry

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